Stack molds have been in use for a number of years to produce injection molded parts of various types, including plastic containers for the packaging industry. In this type of molding operation, a molding press is operated hydraulically and/or mechanically to open and close the mold and hold it closed while molten plastic is injected into the mold cavities. Standard industry practice is to make use of two molding interfaces which are brought together when the press is operated to close the mold and separated when the mold is opened to allow ejection of the molded parts from the machine. This type of molding operation is prevalent in the industry and has resulted in standardization of the available molding presses at a size large enough to accommodate the molding components necessary to carry out two level molding in the press.
Two level stack molding achieves double the production rate of single level molding. The die sets which include the core and cavity plates that come together to form the mold cavities can normally be interchanged to accommodate a change over to a different part.
Canadian Patent No. 2,022,060 to Travaglini et al. discloses a four level stack mold representing an attempt to enhance the production rate. In this arrangement, four mold interfaces are employed in order to produce twice as many parts as a two level mold. However, this increased production is achieved at the expense of trade offs that largely negate the benefits in many applications. In particular, using a four level mold system in a standard press has the disadvantage that the mold blocks occupy so much room that their separation when the mold is opened is minimal. Consequently, parts such as tall plastic containers or anything else having significant height cannot be molded in this type of operation. Instead, only shallow parts such as container lids are practical to mold on a four level system because the mold components cannot be separated far enough to allow deeper or taller parts to be ejected from the mold. The ability to mold only shallow parts is a severe limitation which drastically reduces the practical utility of a four level stack mold system.
In a multiple level mold, problems are also encountered in delivering the molten plastic in a balanced manner to each of the mold cavities. Thermal expansion problems are increased when longer flow paths for the plastic are required. While balancing the plastic flow is not particularly difficult in a four level mold because it is symmetrical, this aspect of the molding operation nevertheless becomes more complex and potentially troublesome with an increased number of molding components.